test_series: fix checkout with ambiguous sha1

Created by: qperret

The current implementation of test_series checks out a kernel based on the first 7 characters of its sha1. However, this truncation creates an ambiguity whenever 7 characters aren't enough to identify a unique kernel. In this case, an error is raised.

This commit fixes this behaviour by removing the hardcoded truncature and cutting the sha1 to be checked out up to the first space found in the series file.

Signed-off-by: Quentin Perret quentin.perret@arm.com